The limitations of conventional lead-acid batteries lie in the electrodes, and a fundamental breakthrough replacing lead with carbon-graphite foam allows the true potential of this mature battery chemistry to be more fully harnessed while maintaining conventional form factors for military equipment.

From the time that the vast and far-ranging Union and Confederate armies of the American Civil War exploited the advantages of instantaneous, long-distance communication by electric telegraph, batteries have played a key role in military technology. Civil War telegraph engineers had to rely on the Grove cell, which wasn't rechargeable and emitted hazardous nitric oxide gas. Despite the drawbacks of that early technology, the military has relied on storage batteries ever since.

When the Civil War began in 1861, the rechargeable lead-acid battery was in its infancy; having been invented just two years earlier by Gaston Planté of France. Remarkably, the basic chemistry and functionality of lead-acid technology have remained essentially unchanged.

In the intervening 148 years, batteries have come to play a variety of key roles in all branches of the world's military forces. Communications remain an important application, as they were in the 1860s. It was, however, with the advent of mechanized warfare in the first half of the 20th century that batteries acquired a new significance as an essential part of the modern military inventory.

The logistical need for standardization has made the 6T lead-acid battery the workhorse for some 95% of U.S. military vehicle applications. In addition to vehicles, the 6T is used in generators and a variety of other support equipment requiring a large-capacity, stable power supply. The U.S Army currently spends approximately $75 million annually on 6T batteries alone.

Battery reliability in military applications is critical. Lives depend on it. At the same time, these applications place demands that push the nearly 150-year-old lead-acid technology to the limits of performance and beyond. Punishing conditions of severe vibration, temperature extremes, and prolonged idle states place extraordinary demands on these batteries. Furthermore, the logistics costs stemming from the weight as well as the need for frequent battery replacement are formidable. According to a recent National Research Council report, it required an estimated $500 million to ship (not purchase) the batteries used in Iraq during Operation Desert Storm, waged from 1990 to 1991. Today, it costs $35,000 per ton to air-ship supplies to a combat theater. The need for a better battery technology is obvious. (Shown in Figure 1 is a battery stockpile in Iraq from the most recent conflict).

A breakthrough technology patented by Firefly Energy is being developed to provide the U.S. military with a cost-effective alternative to conventional lead-acid technology. Without altering the basic lead-acid chemistry, the technology developed by Firefly has shown such promise that the U.S. military has so far contracted for more than $5 million to enable Firefly to adapt the battery technology for stringent military vehicle applications.

The key to Firefly's technology lies in the replacement of the solid lead grids in lead-acid batteries with something new. In place of the conventional grids, Firefly's Microcell technology uses a carbon-graphite material having an extremely porous “foam” structure with hundreds or thousands (depending upon the chosen pore diameters) of spherical cells (shown in Figure 2). The new grid material is impregnated with a slurry of lead oxides. This composite material is then formed up to the sponge lead and lead dioxide in the normal fashion. This creates a unique new cell architecture in which the electrolyte is redistributed from the smaller separator reservoir of a conventional battery to the pores of the foam plates themselves.

The primary advantage of this new structure is that it yields greatly enhanced utilization levels of the lead-acid chemistry, because each microcell has its full complement of sponge lead or lead dioxide and sulphuric acid electrolyte. Liquid diffusion in conventional lead-acid batteries occurs in a single direction along direct pathways on the order of millimeters. Within the discrete microcells that collectively make up a new type of electrode structure (what Firefly calls a 3D electrode), liquid diffusion occurs in all directions along direct pathways on the order of microns. Such a structure results in much higher power, greater energy delivery, and faster recharge capabilities relative to conventional lead-acid batteries.

The use of porous, three-dimensional carbon graphite composite material to replace the lead metal grids in either flooded or valve-regulated (sealed) lead-acid battery designs will release the full power potential of the lead-acid chemistry for energy storage. This delivers a formidable jump in specific power, energy and cycle life. Firefly's advanced battery technology yields performance comparable to advanced materials batteries (lithium and nickel), but can be built for a cost similar to conventional lead-acid batteries, which is one-fifth the cost of advanced materials technology.

Firefly is concurrently developing two versions of its advanced technology. The first of these, “3D,” uses Microcell Technology for the negative plate while maintaining a conventional lead-metal positive plate. The more advanced technology, dubbed 3D2, uses the carbon-graphite foam composite material for the positive and negative plates within the battery. For the purposes of this article, the term “3D” is used to generically describe the carbon-graphite composite cell structure.

Microcell power density attributes

Although lead-acid batteries are the least costly rechargeable energy storage products in the world to manufacture, the extensive use of lead gives them an exceptionally large footprint and weight. This limits their form factors and overall usability in new product designs and advanced applications. In addition, most lead battery plates (more than 100 in a typical large lead-acid battery) only use 20% to 40% of their surface area during each discharge over the life of the battery. This creates even more inefficiencies in power-to-weight ratios. Microcell technology directly addresses this issue in two ways. First, by eliminating most of the lead within the plates themselves, substantial weight reductions are achieved. Second, the greater efficiency and material-utilization characteristics of the carbon-graphite foam cells allow the plates to be much smaller than conventional lead-acid battery cells with comparable output.

In 3D2 technology, volume and weight reductions of as much as 50% relative to comparably performing conventional lead-acid batteries are projected (Figure 3). This approaches the power density levels of much more expensive advanced materials batteries such as NiMH or Li-ion.

Microcell cold temperature attributes

Lead-acid batteries are severely affected by cold temperatures. This is due to an increase in internal resistance caused by a “slowing down” of the battery's chemical reaction and ion diffusion rates. As a rule of thumb, reaction rates are cut in half for each 10 °C drop in temperature. “Cold cranking” is a discharge that needs a high current, meaning a lot of active material conversion in a short time. This is directly related to the amount of electrode surface area covered with active material that is available for conversion.

Sizing a lead-acid battery for starting applications at -18 °C, for example, requires an approximate 200% size increase over a battery with the same capacity at room-temperature operation. A battery designer might contemplate making the lead metal grids thinner, thereby increasing the amount of chemistry that could be placed in a given-size battery box. However, because of an acknowledged corrosion rate for the positive lead grids in lead-acid batteries, trying to increase cold temperature starting power by increasing electrode surface area, without “sizing up” the overall battery, results in severely compromised warm-temperature longevity.

Firefly's 3D and 3D2 products have outstanding discharge performance at low ambient temperatures relative to commercial flooded lead-acid and valve-regulated lead-acid (VRLA) batteries. This is due to the extremely high available surface area of the carbon-graphite foam material. At high discharge rates or low temperatures, the discharge performance of a typical lead-metal-based negative plate limits a cell's output, due in large part to the relatively low surface areas of conventional planar negative plates. The Firefly Microcell electrode is ideal for discharge (and charge) conditions where electrolyte diffusion is limited by surface area, diffusion distances or temperature. Diffusion rates at low temperatures are reduced in a 3D cell just as they are in conventional lead-acid batteries. As previously discussed, however, the distances traveled to react with the sponge lead are much smaller for the new cell architecture. This enhanced electrolyte supply also results in higher, flatter voltage-time curves on discharge, which means higher energy outputs when combined with the lower current densities that accrue from the large available surface area of the foam. This is reflected in Table 1.

As the temperature is lowered, the power required to start an internal combustion engine increases (due to increased oil viscosity), while at the same time the power available from the battery drops, specifically to 40% of what can be provided at ambient temperature for the case when the car is started at -18 °C. By comparison, Firefly 3D battery technology can provide 69% of its ambient-temperature power at -18 °C. This means that an engine-start battery based on Firefly's 3D technology could be sized smaller to have the same cold-crank amps, or it would be more powerful and last longer if its size were comparable to a conventional lead-acid battery.

Microcell hot temperature attributes

The optimum operating temperature for a lead-acid battery is 25 °C (77 °F). As a rule of thumb, every 8 °C to 10 °C rise in temperature will accelerate the corrosion rate of the lead metal grids by a factor of two and result in premature failure for the battery. This is a simple calculation based on field observations and on the increased chemical activity at higher temperatures. Lead grids corrode in the acidic electrolyte in the presence of lead dioxide, the positive-plate active material. High ambient and operating temperatures accelerate this process.

Firefly's 3D battery technology has superior performance in terms of thermal management. The heat-transfer characteristics of the carbon-graphite foam are better than metals such as aluminum and copper, and approach that of a diamond. Figure 4 shows thermal images taken of a Firefly 3D cell and a comparable commercial VRLA lead-acid cell to illustrate further the heat-transfer superiority of carbon-graphite foam. Both cells were subjected to a 5-C-rate (12 minute) discharge, with thermal images taken every 15 seconds. The colors correspond to temperatures above ambient, with the green near or at ambient and the white some 10 °C above ambient.

From Figure 4 it can be seen that the Firefly cell runs cooler overall and the temperature gradient down the negative foam plate is more uniform than for the conventional VRLA cell. This is true even though the Firefly cell's discharge lasted about 2.5 minutes longer. More interesting, this temperature scan shows that the conventional positive plate used in the Firefly 3D cell has a cooler, more uniform heat signature throughout the discharge relative to the other VRLA cell's positive plate, again illustrating the outstanding heat-transfer performance of the negative foam electrode. It not only dissipates the heat generated on itself, but also absorbs heat away from the positive plate and out of the cell. While not shown, the same will be true during recharge and on float, thus suggesting that where positive grid corrosion is the failure mode, lifetimes will be longer in Firefly's 3D technology. It will also make ultrafast recharging more feasible for 3D batteries.

The thermal response patterns for these materials mean that graphite's heat-transfer performance is outstanding. Thus, batteries made with graphite-foam electrodes will transfer heat out of the battery rapidly, as it is generated by the electrochemical reactions taking place, thus making thermal runaway less likely and enabling overall “cool” operation compared to conventional lead-acid batteries. The fact that heat is generated more uniformly and dissipated rapidly translates to longer life in many applications. In the military context, this reduced heat signature reduces the likelihood of infrared detection and makes the battery highly suitable for stealth applications.

Microcell cycle-life improvements

The full discharge of a conventional lead-acid battery causes extra strain, and each cycle robs the battery of a small amount of capacity. In lead-acid batteries, deeper discharges convert larger amounts of charged active-material into lead sulfate. Lead sulfate has a significantly larger volume (about 37% more) than the charged material, and this volume change stresses the electrode structures. This expansion induces mechanical forces that deform the grid, and ultimately result in the lead grid “disappearing” into the paste.

The resulting expansion and deformation of the plates also causes active material to separate from the electrodes with a commensurate loss of performance. Additionally, over time, sulfate crystals can grow together, resulting in larger crystals that are difficult or impossible to convert back into the charged state. This wear-down characteristic also applies to other battery chemistries in varying degrees. To prevent the battery from being stressed through repetitive deep discharge, a larger lead-acid battery and shallower discharge are typically recommended. Depending on the depth of discharge and operating temperature, the sealed lead-acid battery provides 200 to 300 discharge/charge cycles. Short cycle life also results from grid corrosion of the positive electrode, which undergoes extensive oxidative stress during extended recharge conditions. These changes are exacerbated at higher operating temperatures.

In contrast, Firefly's composite plate technology provides a design that fully accommodates the volume changes of the active material during charge and recharge. Within each Firefly plate is contained a full complement of active materials, electrolyte, and volume that will allow complete discharge without causing physical stress on the plate itself. This results in an electrode plate that does not undergo volume change during deep discharges. Firefly's electrode material is not reactive in the lead-acid chemistry and so does not corrode. This is in part due to a natural stability of the base material, but is also due to the formation process used that maximizes exposure to the most chemically resistive surfaces and minimizes exposure to the most chemically reactive surfaces.

The growth of large sulfate crystals is also restricted, resulting in a low incidence of crystals that are too large to dissipate upon recharge. The strong resistance of Firefly's electrode material to corrosion also severely reduces the deleterious effects of long recharges. Because of the significant reduction in these life-limiting factors, the Firefly approach offers significant improvements over conventional lead-acid technologies in float and deep-cycle applications.

Microcell sulfation resistance

Storing lead-acid batteries for extended periods of time, in a fully or partially discharged state, results in a condition known as sulfation. Sulfation is caused by the growth of large sulfate crystals on the battery's negative plates during periods of disuse or dormancy. In a conventional battery, these crystals prevent the battery from being recharged. In the Firefly 3D cell, sulfation reversal is achieved because the nature of the lead sulfate deposits in 3D cells is fundamentally different from those in traditional lead-acid cells. In the latter, lead sulfate is deposited on the surfaces of the plates in dense layers of relatively large crystals, somewhat remote from the lead-grid members. Because the sponge-lead active material in a 3D cell is deposited on the walls of the many small carbon/graphite pores in thin layers, and the high surface area in the collective foam structure results in relatively low current densities, the lead sulfate deposits are comprised of small, porous crystal structures (on the order of three microns to 10 microns) that are easily dissolved on the subsequent recharge. Moreover, these very small crystal sizes grow only slowly over time. This resistance to the effects of sulfation makes Firefly 3D battery technology ideal for military applications where devices and their associated batteries may go unused for months on end, often in a partially or fully discharged state. Conventional batteries are difficult or impossible to recover from these conditions, and are often replaced far short of their potential life span. With 3D technology this problem is greatly reduced.

The low self-discharge rate and easy recovery from sulfation also mean that 3D battery technology is not subject to the shelf-life time constraints of conventional lead-acid batteries. Firefly's 3D battery technology can be subjected to much longer periods of inactivity without damaging effects. Conventional lead-acid batteries are limited to storage times of three to six months at most before requiring a recharge, often with great logistical difficulty and expense.

Microcell vibration resistance

A final life characteristic of the 3D cells is that, because of its use of lightweight carbon-graphite foam, their low mass makes them highly resistant to vibration. Vibration-induced failure is generally sudden and catastrophic, as it often results in a plate breaking loose and short-circuiting a cell. 3D cells subjected to vibration testing at Caterpillar's Technology Center have exceeded Caterpillar's stringent specifications by a wide margin, at which point they still had not failed. Clearly, foam robustness under the abusive conditions found in military applications will not be an issue for Microcell technology.

Microcell safety attributes

While Firefly's technology can bring performance levels close to those of advanced materials batteries such as lithium and nickel, it also has safety advantages over these battery chemistries. This is because the lead-acid chemistry used in Microcell technology is much better understood and, therefore, is more predictable in operation, because it has been in use for 148 years. For example, a concern regarding lithium batteries is that they are subject to violent thermal runaway in the event of a container breach. Once that happens, they do not need a spark for thermal runaway to commence. All that is required is contact with moisture in the air to set up a reaction. Once the reaction begins and the materials within the battery ignite, neither water nor conventional extinguishers will put it out. The materials will burn at 700 8F until they are consumed. This is a frightening scenario in any context, but more so when the inherent hazards of military use are considered.

Preliminary and future military applications

The U.S. Army's high level of interest in Firefly's technology is currently centered upon the Silent Watch program, which involves powering mobile electronic eavesdropping equipment for extended periods without the benefit of the vehicle's charging system (since the engine must be switched off to avoid detection). While this is an ideal application, the overall benefits of this new technology could lead to its use in a variety of other military roles within a few years.

ABOUT THE AUTHOR

Dan Jurchenko is director, defense markets for Firefly Energy. Prior to joining Firefly Energy, Jurchenko spent 12 years with Lockheed Martin, in the business development organizations of three technology centers where he delivered new orders in the electronics area.

Kurtis Kelley is chief technology officer at Firefly Energy. As co-founder, Kelley had previously been a senior research scientist in the Advanced Materials Technology division of Caterpillar's center for research and was responsible for developing and applying materials and design solutions to corporate challenges. He holds more than 25 patents in electronics and material sciences discoveries.